Temperature dependence of fluorescence and phosphorescence of the triphenylamine dimer 3-methyl-TPD
Introduction
Thin films of the aromatic diamine molecule N,N′-diphenyl-bis(3-methylphenyl)-biphenyl-4,4′-diamine (also named N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)-benzidine, abbreviated by TPD or more precisely by 3-methyl-TPD) are widely used as hole-transport layers (HTL) in organic light emitting diodes (OLEDs) [1], [2], [3], [4], [5], [6], [7], [8], [9]. TPD is a laser active material. Whispering-gallery-mode lasing of a TPD coated optical fibre was reported [10]. Travelling-wave lasing of TPD solutions and neat films was achieved [11]. Laser action of TPD based conjugated and non-conjugated polymers was achieved [12], [13], [14].
4-Methyl-TPD (N,N′-bis(4-methylphenyl)-N,N′-diphenyl-benzidine) also works as thin film laser material [15]. Laser action of a copolymer of 4-methyl-TPD with phenyl substituted MEH-PPV was achieved [15]. Recently laser action was shown on un-substituted TPD (N,N,N′,N′-tetraphenylbenzidine) [16].
TPD is used as host material for phosphorescent guest molecules like PtOEP and Ir(ppy)3 [17], [18]. In the case of excitation into the TPD host absorption region efficient Förster-type energy transfer [19] from TPD to Ir(ppy)3 and PtOEP was observed [17], [18], [20]. Dexter-type [21] triplet–triplet forth and back excitation transfer between TPD and Ir(ppy)3 and triplet–triplet forward transfer from TPD to PtOEP were revealed [17], [18], [20]. A numerical simulation of the temperature dependence of the photoluminescence of Ir(ppy)3 in TPD due to triplet–triplet Dexter-type energy transfer is given in [22].
In TPD doped thin polystyrene films efficient Förster type energy transfer from singlet excited polystyrene to TPD was found when the excitation wavelength was in the polystyrene absorption wavelength region [22].
In this paper, the luminescence behaviour of a neat thin film of TPD and of thin films of 5 wt-% TPD in the hosts polystyrene (PS), 4,4′-N,N′-dicarbazole-biphenyl (CBP), and bisphenol-A-polycarbonate (PC) is studied over a wide temperature range from about 10 K to room temperature. The structural formulae of TPD and the hosts PS, CBP, and PC are shown in Fig. 1. The absorption cross-section spectra of TPD and of the hosts are shown in Fig. 2.
Polystyrene is used in OLEDs fabrication as host polymer for forming high quality thin films with good mechanical properties [20], [23], [24], [25]. The absorption and fluorescence behaviour of polystyrene is investigated in [20]. CBP is used in OLEDs as host material for triplet emitters [26], [27]. The fluorescence spectrum and phosphorescence spectrum of CBP are found in [27], [28]. Polycarbonate (bisphenol-A-polycarbonate) is an important plastic material known for its excellent transparency and high impact resistance [29]. It is applied as OLED substrate [30]. The fluorescence behaviour of bisphenol-A-polycarbonate is studied in [29].
The TPD neat films and the TPD doped films were prepared by spin-coating. The temperature dependent luminescence behaviour is compared between untreated films (stored in the dark under ambient conditions till measurement) and annealed films (stored in the dark under ambient conditions and heated to 80 °C for 1 h before measurement). The luminescence spectra are composed of fluorescence contributions and phosphorescence contributions. These contributions are separated out in an analysis of the luminescence spectra. The radiative phosphorescence lifetimes are calculated by applying the Strickler–Berg formalism to the singlet–triplet absorption and phosphorescence spectra. Phosphorescence lifetimes are then estimated from the radiative phosphorescence lifetimes and the phosphorescence quantum yields.
The intention of this paper is to determine absolute fluorescence quantum yields and absolute phosphorescence quantum yields of a 3-methyl-TPD in neat film and of 3-methyl-TPD doped into polymeric hosts (polystyrene, polycarbonate) and in a molecular host (CBP) as a function of temperature. A luminescence analysis method is developed to separate out approximately the fluorescence quantum distributions and the phosphorescence quantum distributions from quantitative luminescence quantum distribution measurements. Phosphorescence lifetimes are estimated from singlet–triplet absorption data and phosphorescence spectra by applying the Einstein relations between absorption, stimulated emission and spontaneous emission [31], [32]. A neat film of TPD was studied to get information about possibly temperature dependent luminescence self-quenching. For the doped films a concentration of 5 wt-% TPD was selected as a typical doping level used in OLEDs [27]. For TPD in polystyrene four samples with molar mass spanning the range range Mw = 2000–500 000 g mol−1 were investigated since a first studied untreated film with Mw = 2000 g mol−1 (containing some residual tetrahydrofuran solvent from preparation) exhibited an unexpected luminescence discontinuity at a temperature of about 150 K, and the reason for this discontinuity was intended to find out. An excitation transfer from host to guest in the case of excitation into the shorter wavelength absorption region of the host is monitored.
Section snippets
Experimental
The organic hole-transport materials TPD and CBP were purchased from Sensient Imaging Technologies, 06766 Wolfen, Germany. The polystyrenes of different molar masses (weight-average molar masses Mw = 2000, 20 000, 100 000, and 500 000 g mol−1) and the solvents tetrahydrofuran (THF, melting point 165 K, boiling point 64 °C) and dichloromethane (melting point 175.7 K, boiling point 40 °C) were bought from Sigma–Aldrich, Germany. Polycarbonate (bisphenyl-A-polycarbonate, Mw = 30 600 g mol−1) was ordered from
Results
The absorption cross-section spectra, σa(λ), of TPD, PS, CBP, and PC are depicted in Fig. 2. Additionally shown is 0.05 × σa(λ) for TPD in order to indicate the absorption strength of 5 wt-% TPD compared to the absorption strengths of the hosts (95 wt-%). The host absorption dominates over the guest (TPD) absorption for λ < 354 nm in the case of CBP, for λ < 309 nm in the case of PC, and for λ < 289 nm in the case of PS.
Luminescence quantum distributions of the investigated thin films are shown in Fig. 3,
Discussion
The luminescence behaviour of a TPD neat film and of TPD doped films on glass substrates was measured as a function of temperature both for untreated films as obtained by spin-coating and for annealed films heated up to 80 °C to remove solvent remaining in the spin-coated films from the preparation process.
At room temperature the remaining solvent in the films was beneficial for an increase of luminescence efficiency. With decreasing temperature in most cases the luminescence efficiency of the
Conclusions
The luminescence behaviour of neat TPD films, and of TPD doped dicarbazole-biphenyl, bisphenol-A-polycarbonate, and polystyrene films was studied over a wide temperature range from 10 K to room temperature. Some remaining solvent from the spin-coating process enhanced the luminescence efficiency at room temperature compared to heat treated films, but at low temperature mostly higher luminescence efficiencies were obtained for the annealed films.
A spectral separation method was developed to
Acknowledgements
The authors are grateful to Anja Merkel for excellent technical assistance. A. P. thanks Prof. F. J. Gießibl for his kind hospitality. T.T. thanks Hideyuki Murayama for his assistance in the temperature dependent photoluminescence measurements at the early stage of the present work. This work was partly supported by the Grant-in-Aid for the Scientific Research from the Japan Society for Science Promotion (Project No. 19560018) and the Research Grants from Kyoto Sangyo University.
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